1. Field of the Invention
The present invention is directed to a method for image processing proceeding from a computer tomography (CT) image of a lung registered with a contrast agent, we well as a CT apparatus for the implementation of such a method.
2. Description of the Prior Art
Computed tomography is increasingly acquiring significance for the diagnosis of pulmonary embolisms. CT angiographies of the vessels in the thorax are analyzed.
For diagnosis of pulmonary embolisms (PE), a CT angiography of the lung is implemented. Contrast agent is injected into the patient with a contrast agent pump. After pausing a few seconds, a spiral CT of the lung is implemented, a stack (series) of axial images, i.e. transverse tomograms, preferably representing body slices adjacent to one another and following one another in the direction of the longitudinal patient axis are acquired. The vessels filled with contrast agent can be clearly seen in CT images on the basis of the increase in density. The diagnosis ensues with reference to the axial images. Dependent on the orientation of the vessels relative to the plane of section, vessels are presented as bright lines or bright points in the so-called lung window (central value of the window=−600 HU; width of the window=1500 HU). The overall vessel tree is tracked when “leafing ” through the image stack and is checked for closures (thrombosis). Blockages or constrictions can be seen as dark matter in the vessel. Following the blockage, the vessel is no longer filled with contrast agent or is only partially filled with contrast agent and is therefore presented darker. This standard technique has the following problems:                Blockages can be overlooked;        The hemodynamic effect of the thrombosis cannot be identified;        Blockages at small vessels (sub-segmental) cannot be recognized in the CT image due to the limited resolution;        The degree of stenosis, i.e. the seriousness of the blockage, cannot be dependably determined; and        Other pathological changes can be erroneously diagnosed as a thromboembolic blockage (for example, closed bronchi).        
U.S. Pat. No. 5,687,209 discloses a method wherein CT images are generated with the assistance of a contrast agent, resulting in that those regions of the image wherein contrast agent is present exhibiting intensified gray scale values, whereas the other image regions are presented with the original gray tones.
U.S. Pat. No. 6,083,162 discloses generation of an interactive three-dimensional presentation of hollow organs under certain circumstances with the assistance of contrast agents, to which end the voxels belonging to the organ to be presented are separated from the rest of the image content.